Terpene recovery process including control of a digester relief outlet

ABSTRACT

IMPROVED BATCH KRAFT DIGESTION OF WOOD CHIPS IS OBTAINED BY VENTING DIGESTER VAPORS THROUGH A VALVE AT MAXIMUM OPENING SHORTLY AFTER DIGESTER HEATING IS INITIATED, THEN GRADUALLY REDUCING THE VALVE APERTURE WHILE MAINTAINING DIGESTER PRESSURE ASCENDING GRADUALLY TO A VALUE BETWEEN ABOUT 80 AND ABOUT 120 P.S.I.G., AND THEREAFTER MAINTAINING SUBSTANTIALLY CONSTANT RELEIF FLOW THROUGH THE VALVE   AND SUBSTANTIALLY CONSTANT DIGESTER PRESSURE UNTIL TERPENES ARE SUBSTANTIALLY DEPLETED FROM THE RESULTING PULP.

R. E. MARKS OF A DIGESTER RELIEF OUTLET Filed Oct. 2, 1972 u Separator TERPENE RECOVERY PROCESS INCLUDING CONTROL June 18, 1974 '2. in hours IFIG;

Sfeam for B/owbaa/r Time United States Patent US. Cl. 16261 1 Claim ABSTRACT OF THE DISCLOSURE Improved batch kraft digestion of wood chips is obtained by venting digester vapors through a valve at maximum opening shortly after digester heating is initiated, then gradually reducing the valve aperture while maintaining digester pressure ascending gradually to a value between about 80 and about 120 p.s.i.g., and thereafter maintaining substantially constant relief flow through the valve and substantially constant digester pressure until terpenes are substantially depleted from the resulting pulp.

This invention relates to the batch digestion of wood chips with aqueous kraft cooking liquor, and more particularly to improved venting operations for such digestion.

In such wood pulp digestion it is customary to charge the digester with wood chips and usually a mixture of white and black liquors as the cooking liquor to cover the chips. The digester is run on an approximately 1 to 3-hour cycle or longer. Peak pressure generally is approximately 80-120 p.s.i.g., the at pressure time being about 20 minutes to an hour. The digester batch can be heated indirectly, or preferably directly for economy by injecting open steam into the batch, or by a combination of direct and indirect heating. The heating is done at a rate to drive the digester pressure fairly rapidly to the maximum value with safety and to hold it there until digestion is completed. Non-condensable gases such as air, some sulfur compounds, steam, and turpentine are vented from the digester. Turpentine can be recovered typically as shown in US. Pat. 3,607,617 of John Drew.

Heretofore perhaps the most eiiicient scheme proposed for vent relief with terpene recovery involved passing the outlet vapors from such digestion through a restricted orifice until the digester pressure reached approximately 50-60 p.s.i.g., then running such vapor stream through a substantially enlarged orifice when the bulk of terpene vapors (turpentine) were being emitted; the opening in the vent relief line thereafter could be reduced to save process steam. Such operation and other conventional methods of controlling the vent relief are described in the text Sulfate Turpentine Recovery, edited by Drew et al. and published in 1971 by Pulp Chemicals Association, New York, N.Y., at pages 68-71.

By use of the instant improvement in the venting of the digester substantial process (heating) steam savings have been experienced, increased turpentine production has been indicated, and a consistent good quality of pulp has been obtained without creating production problems in digester performance.

This invention is an improvement in a process for digesting wood pulp kraft cooking liquid in a digester wherein the vapors from said digester are vented through a relief outlet for terpene recovery. It comprises:

initiating digestion heating at substantially atmospheric pressure in a closed digester;

at digester pressure not substantially in excess of about p.s.i.g., opening to a maximum for said cook a relief valve communicating with said relief outlet;

gradually reducing the relief valve opening responsive to maintaining digester pressure ascending gradually to a valve between about 80 and about 120 p.s.i.g.;

3,817,824 Patented June 18, 1974 "ice thereafter maintaining substantially constant flow through said relief valve and substantially constant digester pressure until terpene values are substantially depleted from said pulp and said pulp is digested, and

interrupting the relief flow through said relief valve during the cook only for periodic blowbacks to clean debris from said relief outlet.

FIG. 1 of the drawings shows plots of digester pressure and relief flow for a typical relief cycle from a 4500 cubic foot batch kraft pulp digester. It will be more fully discussed in connection with the example that follows and is idealized to the extent that blowbacks for cleaning debris from the relief outlet are not indicated.

FIG. 2 of the drawings is an equipment diagram for the operation of such digester. Digester vessel 11 is equipped with an internal live steam heating and sparging line, not shown, temperature indicator 12, pressure indicator 13, digested pulp outlet valve 22, and a nominally 3-inch I.D. relief line 14 venting the digester into a gas-liquid-pulp solids separator (not shown). To prevent excessive amounts of wood chips, pulp, and/or other debris from going through relief line 14, there customarily is a coarse screen in the apparatus where relief line 14 joins the top section of the digester. This screen is not shown. It ordinarily has about 5 apertures. The digester is charged from the top with wood chips and cooking liquor.

Valve 17 is a heavy-duty, nominally 2-inch high-pressure control valve, typically a V-port ball valve operated pneumatically with elevated instrument air pressure acting to open it wide and diminishing instrument air pressure throttling it down substantially proportionally to decrease in such air pressure over the operating instrument air pressure range. The output signal that regulates instrument air pressure to valve 17 is supplied from a pressure tap on the digester so that valve 17 is being closed gradually, and responsively to maintaining digester pressure ascending gradually to an at pressure valve between about -120 p.s.i.g.

Valves 15 and 16 are hand valves for manual blocking of relief line 14. Normally during a cook they are left open. Valve 18 is an automatic block valve set to open shortly after the beginning of the cook when the digester pressure reaches a preselected value not above about 10 p.s.i.g. and generally 4-5 p.s.i.g. It is set also to shut off periodically during a cook, while at the same time automatic valve 19 opens fully (along with control valve 17) so that blowbaok steam at -200 p.s.i.g. can enter line 21, then go through line 14, valves 17 and 15, blowing away fibrous material and other debris from those valves, that line, and oif the screen at the digester vapor outlet. The valves 18 and 19 also are set to perform the same automatic blowbaok operation (with the concomitant opening wide of "valve 17) should the pressure in the digester reach 5-7 pounds above that corresponding to the pressure of saturated steam at the digester temperature, this latter setting being a control for the steam blowback that overrides the periodieblowback (which, for example, can be set for intervals of 10-15 minutes, more or less as is needed). Digester pressure throughout the cook corresponds fairly closely with that of saturated steam at the digester temperature.

At the end of the run (cook) valve 22 is opened, valve 18 closes automatically, and the processed charge blown by internal pressure of vessel 11 into a tank not shown.

The following example shows how the invention has been practiced, but should not be construed as limiting it.

EXAMPLE 1 The digester used was a 4500 cubic foot, batch digester charged in conventional fashion with wood chips and cooking liquor for kraft pulp digestion. The cook described had essentially the relief cycle characteristics of digester pressure and vent gas flow rate shown in FIG. 1. The digester and vent apparatus and operation were those described in connection with FIG. 2.

At the beginning of the run pressure built up autogenously in the digester to about 5 p.s.i.g. from atmospheric. At this point, valve 18 (which was closed) opened. Valve 17 was wide open. Valve 18 stayed open throughout the balance of the cook except for occasional blowbacks of high pressure steam as described with respect to FIG. 2. The control output of air pressure operating automatic valve 17 gradually closed it so the valve opening had approximately the following characteristics: at 4 minutes it was 100% open; at minutes, about 90%; at 20 minutes, about 70%; at minutes, about 50%; at minutes, about 37% and at minutes, about 32%.

The digester pressure gradually ascended as indicated from atmospheric at zero time to 110 p.s.i.g. at 90 minutes and thereafter was kept substantially constant until the end of the cook. The vent relief flow started at about 4 minutes when valve 18 opened. After a short surge rich in non-condensables, the relief flow ascended gradually in the pattern that was essenially proportional to the digester pressure, said relief flow reaching a maximum of about 900 pounds per hour at about 67 minutes. At this point relief flow was maintained substantially constant toward near the end of the run. The cook was terminated shortly after two hours, and the digested charge blown out of the digester and into a blow tank.

The operation showed large process steam savings over previous experience with small-to-large-to-small orifices used as described hereinbefore. Also it was noted that substantal turpentine started to flow in the vent relief cycle at 15-25 p.s.i.g. digester pressure (rather than the higher p.s.i.g. so experienced with such orifices). Addi- 35 tionally it appeared that as much as roughly about 7% more turpentine was recovered.

I claim:

1. In a process cook wherein a batch of wood chips is heated for digestion to pulp with a kraft pulping liquor in a digester and the vapors from said digester are vented through a relief outlet for the recovery of terpene values, the improvement which comprises:

initiating digestion heating at substantially atmospheric pressure in a closed digester; at digester pressure not substantially in excess of about 10 p.s.i.g., opening to a maximum for said cook a relief valve communicating with said relief outlet;

gradually reducing the relief valve opening responsive to maintaining digester pressure ascending gradually to a value between about and about p.s.i.g.;

thereafter maintaining substantially constant flow through said relief valve and substantially constant digester pressure for a time suflicient to complete digestion, and

interrupting the relief flow through said relief valve during the cook only for periodic blowbacks to clean debris from said relief outlet.

References Cited UNITED STATES PATENTS 1,025,356 5/1912 Williamson 162-15 1,814,027 7/ 1931 Dunbar l62-61 X 2,996,423 8/1961 Nyquist 16252 3,183,145 5/1965 Collins, Jr. 162-15 X 3,492,198 l/1970 ROScnblad 162-15 S. LEON BASHORE, Primary Examiner R. V. FISHER, Assistant Examiner US. Cl. X.R. 16215, 239 

